Received this from a friend.
Window's habit of breaking timing critical I/O is well enough known 
but this may be of interest to people wishing to bit bash USB I/O


        RM

_________


Russell,

The FTDI USB-serial bridge devices have a "bit-bang" mode whereby 
bytes sent
to the output FIFO are clocked out in parallel at the selected bit 
rate.
For instance if the all outputs are currently high and you send the 
sequence
7Fh, 7Eh, 7Fh, FFh you will generate nested low-going pulses on the 
MSB and
LSB output pins:


            |        |        |        |        |        |

            ---------                            ---------
 MSB                 \                          /
                      --------------------------

            ------------------           -----------------
 LSB                          \         /
                               ---------


This mode was originally intended for implementing synchronous serial
interfaces such as SPI, I2C, JTAG, and those present on SRAM-based 
FPGA's
for loading the logic definitions, but obviously has other potential
applications.  The above waveforms might for instance be used as the
chip-enable and write-strobe for a character-type LCD interface.

There is however a nasty "gotcha" which FTDI have neglected to 
mention.

The implication of all the FTDI documentation is that if you send the 
data
sequence as a single API write that all bytes will arrive at the 
bridge
device and be clocked out on successive "ticks" of the bitrate clock 
(as
depicted above).

Not so !

The FTDI API (driver) appears to be at the mercy of the underlying 
Windows
USB transport system which arbitrarily decides how many bytes are
transferred in any given USB frame, and in general will break the data
sequence up into N smaller chunks which it sends at 1ms intervals (the 
basic
USB frame interval).  The end result is that any state on the FTDI 
bridge
outputs can be stretched for up to 1ms (and I guess possibly for 
multiples
of 1ms).

In some applications this may not matter but if it does you are in 
trouble.

FTDI have confirmed this behaviour and can offer no workaround.

Regards,

Ken Mardle

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